CLUES TO NUCLEAR STAR CLUSTER FORMATION FROM EDGE-ON SPIRALS

Abstract

We find nine nuclear cluster candidates in a sample of 14 edge-on, late-type galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys. These clusters have magnitudes (MI rv 11) and sizes (reA rv 3 pc) similar to those found in previous studies of face-on, late-type spirals and dE galaxies. However, three of the nuclear clusters are significantly flattened and show evidence for multiple, coincident structural components. The elongations of these three clusters are aligned to within rv10o of the galaxies’ major axes. Structurally, the flattened clusters are well fit by a combination of a spheroid and a disk or ring, with the disk preferred in two of three cases. The nuclear cluster disks/rings have F606W - F814W (rvV - I ) colors 0.3Y0.6 mag bluer than the spheroid components, sug- gesting that the stars in these components have ages <1 Gyr. In NGC 4244, the nearest of the nuclear clusters, we further constrain the stellar populations via spectroscopy and multiband photometry. This nuclear cluster is equally well fit by single stellar populations with ages of either rv70 Myr or rv0.8 Gyr and with masses of rv3 ; 106 MG. However, significantly better fits to the spectroscopy and photometry are obtained by combining two or more stellar populations. Exploiting emission lines that appear to originate rv100 from the NGC 4244 nucleus, we determine a

lower limit on the dynamical mass of 2:5þ1:7 ; 106 M

within19 pc, typical of values found for other nuclear clusters.

We also present tentative evidence that -1:2

r of t

G
he nuclear clusters (in NGC 4206) may also host a supermassive

black hole. Based on our observational results we propose an in situ formation mechanism for nuclear clusters in which stars form episodically in compact nuclear disks and then lose angular momentum or heat vertically to form an older spheroidal structure. We estimate the period between star formation episodes to be rv0.5 Gyr and discuss possible mechanisms for transforming the disklike components into spheroids. We also note the connection between our objects and massive globular clusters (e.g., ! Cen), ultracompact dwarfs, and supermassive black holes.